Amplifier device

ABSTRACT

An amplifier device, including: at least one input contact, an amplifier section for amplifying a signal presented at said input contacts whereby an amplified signal is obtained; said amplifier section being connected to said input contact; a bias current source connected to said amplifier section for providing a bias current which at least partially controls the gain of the amplifier section, at least one output contact connected to the amplifier section, for receiving the amplified signal from the amplifier section and transmitting the amplified signal further. The bias current source is an alternating current source having an adjustable duty cycle.

This application is the US national phase of international applicationPCT/NL01/00529 filed in English on 11 Jul. 2001, which designated theUS. This application claims priority to PCT/NL01/00529. The entirecontents of these applications are incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The invention relates to an amplifier device; a method for operating anamplifier device; and to electronic devices including an amplifierdevice.

2. Related Art and Other Considerations

In amplifier devices control of the gain of an amplifier may berequired. For example in integrated receivers used for the reception ofradio signals it may be necessary to control the gain if a very large(adjacent) channel interference is present in the frequency spectrum ofthe received signals.

It is known in the art to control the gain of an amplifier device in ananalog way by changing the gain determining elements in the amplifierdevice. For example, if the gain is controlled by a resistance, a secondresistance may be placed in series or in parallel, thus increasing orreducing the gain.

It is also known in the art to control the gain of an amplifier devicewith a differential pair, for instance, two bipolar transistors withtheir emitters connected to each other and the collectors formingdifferential output contacts. The desired attenuated signal is thenobtained from one of the differential output contacts while the otherdifferential output contact is disregarded.

Furthermore, it is known to control the gain of an amplifier device byadjusting the DC bias current of the amplifier device. This may forexample be implemented with a bipolar junction transistor (BJT)connected with the collector to an adjustable current source. The biascurrent is then controlled by the voltage over the base of the BJT.

However, each of the known gain controls is disadvantageous because ofone or more of the following reasons. The control curve (i.e. theattenuation as a function of control signal) may be non-linear; theimplementation or design of the device is complicated andtime-consuming; or the generation of the control voltage has to beimplemented in an analog (not digital) way.

BRIEF SUMMARY

An amplifier which reduces at least one of these disadvantages ischaracterised in that the bias current source is an alternating currentsource having an adjustable duty cycle.

Thereby, the gain of the amplifier device may be controlled in arelatively simple manner by tuning the duty-cycle of the AC controlledbias current source. Furthermore, the gain depends in a substantiallylinear manner on the duty-cycle of the bias current. Also, control ofthe duty cycle and thereby control of the amplifier gain may beimplemented with digital means, for example by counting the number ofpulses from a clock. While the number of counted pulses is below a firstpredetermined value N, the output signal is high and if the number ofcounted pulses is above the first predetermined value N but below asecond predetermined value M the output signal is low. The duty cycle isthan given equal to N/M. Furthermore, the gain is linear with theduty-cycle, i.e. a 100% duty-cycle (equivalent to normal DC biasing)gives maximum gain and maximum power consumption, 50% duty-cycle giveshalf the maximum gain, and half the maximum power consumption. Thereby,if the gain is lowered for high-level signals, than the powerdissipation is lowered also. Thereby, the amplifier device is especiallysuited for applications with limited power supply, such as wirelessradios, mobile telephones and bluetooth devices.

Electronic devices including an amplifier device are also providedwherein the gain of such electronic devices may be adjusted by adjustingthe gain of the amplifier device. Furthermore, the gain is linear withthe duty-cycle, i.e. a 100% duty-cycle (equivalent to normal DC biasing)gives maximum gain and maximum power consumption, 50% duty-cycle giveshalf the maximum gain, and half the maximum power consumption. Thereby,if the gain is lowered for high-level signals, than the powerdissipation is lowered also. Thereby, the electronic device isespecially suited for applications with limited power supply, such aswireless radios, mobile telephones and bluetooth devices.

Furthermore, a method for controlling the gain of an amplifier device isalso provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, effects and aspects will be described with reference tothe figures in the drawings.

FIG. 1 shows a block diagram of an example of an embodiment of receiverfront-end device;

FIG. 2 shows a graph of the simulated gain as a function of theduty-cycle period of the bias current of the example of a receiverfront-end device of FIG. 1; and

FIGS. 3–4 show circuit diagrams of examples of embodiments ofamplifiers.

DETAILED DESCRIPTION

FIG. 1 shows a receiver front-end which includes an amplifier 1, a mixer2 and an IF-filter 3. At a first receiver input contact RFp and a secondreceiver input contact RFn a RF signal of radio frequency is received.First, the RF signal is amplified by the amplifier 1 whereby anamplified RF signal is obtained. The RF signal is fed into a positiveamplifier input contact inp and a negative amplifier input contact innof the amplifier 1. A current source BIAS supplies the amplifier with abias current. The amplified RF signal is transmitted via a positiveamplifier output contact outp and a negative amplifier output contactoutn to an mixer input RF of mixer 2.

The mixer 2 mixes the amplified RF signal with a local oscillator (LO)signal of a local oscillator. The LO signal is generated by a LO sourceV0 connected to a LO input lo of the mixer 2. The mixing operationresults in a mixer output signal which is transmitted to the filter 3via a mixer output mout connected to a filter input IF_in. The mixeroutput signal is then filtered by the filter 3, whereby an intermediatefrequency (IF) signal is obtained at a filter output IF_out.

The gain of the amplifier 1 is controlled by the bias current from thebias current source BIAS. As shown in FIG. 1 the bias current sourceBIAS provides a square wave bias current. That is, the bias current haseither a value I₁ or a value I₂. The generation of a block signalcurrent is generally known in the art and the bias current source may beof any type appropriate in the specific implementation. The bias currentsource may either be an analogue or a digital device. For example, thebias signal may be generated with a DC current source connected to oneinput of an edge triggered bistable (also known as flipflop) and a clocksignal supplied to both the second input and the clock signal of theinput. The bias signal may be generated with in any other way, forexample by counting the number of pulses from a clock. While the numberof counted pulses is below a first predetermined value N, the outputsignal is high and if the number of counted pulses is above the firstpredetermined value N but below a second predetermined value M theoutput signal is low. The duty cycle is than given equal to N/M.Furthermore, the bias current may be of any non-DC signal type, such asa sine wave signal, saw tooth signal or any other type of signal.

The gain of the amplifier section is controlled by the duty cycle of thebias current, that is the ratio of the time the bias current has thevalue I₁ and the total signal period. In a mathematical form theduty-cycle is defined as:

$\begin{matrix}{d = \frac{t_{I_{1}}}{t_{I_{1} + {I2}}}} & (1)\end{matrix}$

In eq. (1) t_(I) ₁ is the time the bias current has the value I₁ andt_(I) ₁ _(+I2) is the total period, that is the time signal the biascurrent has the value I₁ and the bias current source has the value I₂.The effective or average bias current is equal to:

$\begin{matrix}{I_{average}^{bias} = \frac{{I_{1} \cdot t_{I_{1}}} + {I_{2} \cdot t_{I_{2}}}}{t_{I_{1} + {I2}}}} & (2)\end{matrix}$

For a low duty-cycle the effective or average biasing current will below. Therefore, the effective or average gain of the amplifier device islow too. For higher duty-cycles, the gain increases. The duty-cycle ofthe amplifier bias current thus controls the gain of the amplifier in asimple manner.

Furthermore, the biasing current may be lowered for high-level signalsby reducing the duty cycle of the bias current signal. Thereby theaverage bias current is reduced. The reduction of the average biascurrent results in a reduction of the power consumption of the circuit.Thereby, the receiver is especially suited for applications with limitedpower supply, such as wireless radios, mobile telephones and Bluetoothdevices.

Also the gain of the amplifier device is substantially linearlydependent on the duty-cycle. FIG. 2 shows a graph of the simulatedoutput signal level of the amplifier device (as averaged by thesimulator) at contact RFb and the signal level at the filter outputcalculated at contact IF (as averaged by the IF-filter) as a function ofthe time the bias current has the value I₁ i.e. the pulse width of thebias current. In the simulation the following parameter values whereused I₂ was set to zero and the mixer was supposed to be an ideal mixer.The frequency of the RF signal was 102 MHz. The amplifier was an RFamplifier in this example providing a gain of some 12 dB. The frequencyof the duty-cycle modulation signal has been 10 MHz. This frequency ishigh enough to ensure that the bandpass filter filters the duty-cyclesignal significantly, and low enough not to pose any stringentrequirements on the biasing circuitry. The duty-cycle was varied from 0%(pulse time 0 ns) to 100% (pulse time 100 ns).

The invention is not limited to the parameter values used in thesimulation, the parameters may be set to any appropriate value. Forhigher frequencies of the bias current AC signal, the spuriousresponses, that is the frequency components in the RF signal notsubstantially equal to the frequency of the RF signal, are moreseparated from the desired operational frequency band and are attenuatedby the RF filter. As can be seen, the pulse width of the bias currentdetermines the gain of the amplifier and the gain of the receiveritself. The gain as a function of the duty-cycle is highly linear forpulse durations between 5 and 100 nanoseconds.

An amplifier with an AC controlled bias current may behave in amixer-like way, whereby the amplified RF signal may include extraspurious frequency components. In the front-end of FIG. 1 this may causesensitivity of the receiver to not only the desired signal, but alsoother signals of frequency substantially equal to the radio frequency ofthe RF signal plus or minus the frequency of the AC controlled biascurrent. Provisions may be made to limit these spurious responses. Forexample, the duty-cycle periodicity of the duty-cycle signal itself maybe modulated, as is known from for example delta-sigma modulators, or apseudo-random sequence may be applied into the duty-cycle signal.Reference is made to S. R. Norsworthy, R. Schreier, G. C. Temes,“Delta-sigma Data Converter, Theory, Design and Simulation”, IEEE PRess,1996. For example, the frequency may be modulated to vary from 9 to 11MHz and the duty cycle may vary as in table 1, whereby as an average thefrequency is 10 MHz and the duty cycle is 30%, that is the signal has30% of the time the value I₁ and 70% of the time the value I₂.

TABLE 1 I₁ I₂ (in % of the period) (in % of the period) 30 70 40 60 2080

Such modulation and variation of the bias signal results in a widerspread of the spurious components over the frequency spectrum. Since thetotal energy of the spurious components is not increased, thefrequencies will be less apparent and the spurious components in thesignal will have increased noise like characteristics. The variation andspread may for example be obtained by using pseudo-random-bit-sequence(prbs) signals, as are generally known in the art of standard testequipment for testing digital transmission data links (Bit Error Ratetests).

Because the amplifier is switched OFF for several RF-periods, some kindof averaging is needed. In a receiver a bandpass filter is usuallyavailable for filtering adjacent channels. In the receiver front-endshown in FIG. 1 the filter 3 averages the converted RF signal, therebyreducing unwanted spurious components. The use of an already availablefilter reduces the number of components in the device and thus bothpower consumption and complexity. However, a separate filter foraveraging the signal may be connected to the amplifier instead.

In the example of a receiver shown in FIG. 1 a non-zero IF filter isused for averaging. In FIG. 1, a low-IF receiver is used, andconsequently the channel filter is a band-pass filter. A zero-IF filtermay also be used. In that case the channel-filter is a low-pass filter,which may be used for averaging in a likewise manner. Furthermore, theother stages in the receiver shown in FIG. 1 may be provided with an ACcontrolled bias current source for gain control. For example the IFfilter may comprise a chain of filter stages or be a cascaded filter, asis generally known in the art. (From filter theory it is known that anyhigh order filter may be replaced by several low order filters). Thegain of the filter may then be controlled by providing one or more ofthe filter stages with an AC controlled bias current and setting theduty-cycle thereof to an appropriate value.

Any amplifier device having a bias current source may be provided withan AC controlled bias current source for control of the gain of theamplifier. In general, class A, A/B and B amplifiers have a bias currentsource. However, the invention is not limited to these amplifier classesand may be used in any type of amplifier.

Examples of amplifier devices having a bias current are shown in FIGS.3–4. FIG. 3 shows a differential amplifier device. The differentialamplifier device includes two n-type Metal Oxide Semiconductor FieldEffect Transistors (nMOSFET) T1,T2 connected to each other an a biascurrent source BIAS with their sources. The drain of each of thenMOSFETs is connected via a resistor R1,R2 to a voltage supply VCC. Thedrain of the nMOSFET T1 is a positive output contact out_p. The drain ofthe nMOSFET T2 is a negative output contact out_n.

FIG. 4 shows a transconductance amplifier including a bipolar junctiontransistor (BJT) T3. The base b3 of the BJT T3 is a positive inputcontact. The emitter e3 of the BJT T3 is both a negative input contactand a negative output contact of the transconductance amplifier. Thecollector c3 is the positive output contact c3. The collector c3 isconnected to a voltage supply VCC via a bias current source Ibias.Between collector c3 and base b3 a DC-feedback loop is provided for thecorrect DC-baising of the transistor, as is indicated with the dottedline DC.

An amplifier device according to the invention may be implemented in anytype of electronic device or electronic circuit, such as a mobiletelephone, a receiver circuit, a home stereo set or measurementinstruments. Implementation of an amplifier device is especially suitedin wireless electronic devices with limited power supply because of therelatively low power consumption of the amplifier device. The wirelesselectronic device may for example be a radio, a mobile telephone orBluetooth devices like bluetooth headsets.

An amplifier device according to the invention may be used to perform again control method. Such a method includes determining a required gainof the amplifier device. This may be any gain required in the specificimplementation of the amplifier device. Thereafter a bias currentcorresponding to the required gain is generated. The bias current is analternating current, having a duty-cycle such that the time average ofthe bias current is substantially equal to the bias currentcorresponding to the required gain.

1. An amplifier device comprising: at least one input contact, anamplifier section connected to said at least one input contact foramplifying a signal presented at said at least one input contact wherebyan amplified signal is obtained; a bias current source connected to saidamplifier section for providing a bias current which at least partiallycontrols the gain of the amplifier section, at least one output contactconnected to the amplifier section, for receiving the amplified signalfrom the amplifier section and transmitting the amplified signalfurther, wherein the bias current source is an alternating currentsource having an adjustable duty cycle, said alternating current sourcebeing directly connected to the amplifier section.
 2. An amplifierdevice as claimed in claim 1, wherein the bias current source is asquare wave current source.
 3. An amplifier device as claimed in claim1, further including duty-cycle modulation means for modulating theduty-cycle to minimize spurious components in the signal.
 4. Anamplifier device as claimed in claim 3, wherein the duty-cyclemodulation means include at least one delta-sigma modulator device. 5.An amplifier device as claimed in claim 3, wherein the duly-cyclemodulation means is controlled by a duty-cycle modulation signal andwherein means are provided for selecting the frequency of the duty-cyclemodulation signal for minimum spurious responses.
 6. A method forcontrolling the gain of an amplifier, comprising: determining a requiredgain of the amplifier; tuning a duty-cycle of an alternating biascurrent such that the time average of the alternating bias current issubstantially equal to the bias current corresponding to the requiredgain, and generating a bias current corresponding to the required gainby generating said alternating bias current having said tunedduty-cycle, which alterating bias current is directly provided to theamplifier.
 7. An electronic device including at least one amplifierdevice as claimed in claim
 1. 8. An electronic device as claimed inclaim 7, further including: at least one mixer device connected to saidamplifier device; at least one filter device connected to at least oneof the mixer outputs.
 9. An electronic device as claimed in claim 7,wherein the electronic device includes: at least one bandpass filterconnected to said amplifier device, said at least one bandpass filterbeing a distributed filter chain including: at least two filter stages,at least one of said filter stages including an AC controlled biascurrent source having an adjustable duty-cycle which at least partiallycontrols the gain of the filter stage.